Identification of free-roaming animals and movable objects is desirable to scientists, ranchers, and persons providing inventory control. The challenge is to provide a convenient means for attaching identification information to a movable object or animal. One simple method is to attach a visible tag to the object or animal with identification data written thereon. However, such tags are easily altered or destroyed, enabling an interloper to claim title to the tagged property.
Therefore, others have developed several ways to conceal tags, implant tags in animals, and provide encoded tag information which is machine-readable but unintelligible to the naked eye. For example, a label can be provided with a bar code, and a bar code reader can be used to read identifying information from the label. Unfortunately, bar code systems can store only a small amount of information, and the bar code can be altered or destroyed. Also, the bar code must be clearly visible for proper reading.
One way to avoid the problem of visibility is to use a sealed tag with identifying information electronically stored in a memory means such as an integrated circuit memory. With such a device, one must provide a means for reading the memory means, since the memory means is concealed from view. Radio transmission could be considered, but its bandwidth is very limited, reducing the speed of transmission and data-carrying capacity; suitable equipment would also require compliance with numerous federal broadcasting regulations. Miniaturization of radio transmitters for implantation in a living animal is also impractical.
Therefore, inductive closed-coupled identification systems have been developed, having a sealed tag and a reader using electromagnetic energy transmitted to the tag. Such inductive systems can include a passive implanted tag with a memory means coupled to an inductive coil which serves as an antenna and facilitates an inductive power supply. A separate tag reader which can include a battery power supply has a field coil for transmitting a high-power electromagnetic field to the tag. The field is received by the tag and converted through induction to a direct current power supply signal to run the tag circuitry. The tag can then retransmit identification data to the reader by reading the tag memory means, and the reader can display the data. These systems permit powering a passive identification tag transponder by an electromagnetically coupled energizer reader, and the transmission of an ID signal through a single coil in the tag. This type of system is disclosed in U.S. Pat. No. 4,703,756, which discloses a battery powered implant which transmits a signal to an external receiver, and also in U.S. Pat. No. 3,689,885 (Kaplan et al.) and 3,869,624 (Kriofsky et al.). A similar approach is disclosed in U.S. Pat. No. 3,706,094 (Cole et al.) which describes an identification tag with a substrate of piezo-electric material with coded information stored therein. Energy transmitted by a reader to the tag is converted into acoustic energy, modulated, reconverted to electromagnetic energy, and retransmitted to the reader.
Unfortunately, all the systems of these prior art devices require means in the tag for retransmitting data. This approach requires use of two transmission-reception channels as well as transmission and reception circuitry in both the reader and the tag. Since it is desirable to miniaturize the tag, especially when the tag must be implanted in an animal, it is desirable to eliminate as many parts in the tag as possible. Conventional systems are also susceptible to interception of signals by undesired observers or listeners, and by interference signals in the environment.
In a prior art reader a single-ended circuit is used such as that shown in FIG. 1A. A single capacitor 10 is employed in series with a single-ended driver coil 12 which together provide a resonant circuit. This type of system is also disclosed in U.S. Pat. No. 4,730,188 (Milheiser), which shows an interrogator coil 14 in FIG. 1 with one side coupled to ground and a signal detection system coupled to the other side at point TSI-7. However, such single-sided systems are highly susceptible to electromagnetic interference because of the ground coupling.
To address these and other drawbacks, a reader and tag system is known which reads tag data by providing a variable loading means in the tag, as disclosed in U.S. Pat. Nos. 4,517,563 and 4,333,072. To decode the data, the reader measures power output and loading by the tag. The modulated power signal is decoded to separate a data element for later digital interpretation. This permits detecting the tag ID signal through a coil in the tag reader by sequentially varying the reader power consumption of the tag in accordance with a pre-determined code number. Signals other than fixed ID numbers may also be transmitted. However, the '072 patent discloses a field coil circuit which does not efficiently generate output power and which has relatively low ripple characteristics. Thus, common mode rejection is average, making the circuit susceptible to electromagnetic interference (EMI).
Providing a rectifier in a tag for conventional power supply voltage rectification is known, as disclosed in U.S. Pat. No. 4,196,418 (Kip et al.) and 4,724,427 (Carroll). Carroll discloses use of a rectifier in a tag of an inductively coupled transponder system; the rectifier provides tag power rectification and provides balanced modulation of the tag ID code into the tag coil. This circuit mixes the modulation signal into the power field of the energizing device rather than causing a variable loading of the tag, and thus operates similar to the active transponder type of tag systems. U.S. Pat. Nos. 3,440,663 and 3,299,424 (Vending) each disclose in FIG. 1 a diode 26 which series as a signal detector. However, the diode simply provides a voltage drop to enable load detection rather than acting as a rectifier. Thus, the Vinding circuits do not provide an increase in ripple frequency.
Therefore, users of inductively coupled identification systems desire to have an efficient high-power reader system which increases the practical distance by which the reader and tag can be operationally separated by radiating maximum output power at the resonant frequency, and which is maximally sensitive to changes in power consumption by the tag at the frequency of information transmission. Moreover, users desire a system which is more immune to EMI, and which extends battery life by consuming less input power. It is also desirable to have a transmission and power consumption measurement circuit in a reader which is inexpensive, and forms improved signals with greater power and ripple.